Should WELS school principals accept gifts from contractors?

Should WELS school principals accept gifts from contractors? Should there be a threshold value separating trivial gifts from substantive gifts?

For example, when school uniform companies compete for the privilege of being designated as your school's uniform provider, do some uniform companies "sweeten the deal" by providing freebies to your principal? If your school organizes overnight class trips through a travel agent, does the travel agent "comp" your principal's travel?